使用 mongodb 和 java

Question

我正在数据库中搜索 URL，但使用此代码我做不到。为什么？通常我想打印此数据库中存在的所有类型和 URL 。当我打印时只有类型有效但打印为 URL 什么都没有。

MongoClient mongoClient;
DB db;

mongoClient = new MongoClient("localhost", 27017);
db = mongoClient.getDB("behaviourDB_areas");    


DBCollection cEvent = db.getCollection("event");

    BasicDBObject orderBy = new BasicDBObject();
    orderBy.put("timeStamp",1);


    DBCursor cursorEvents = null;

    BasicDBObject searchQuery = new BasicDBObject();
    searchQuery.put("user_id", "55b20db905f333defea9827f");

    cursorEvents = cEvent.find(searchQuery).sort(orderBy);

        int count=0;

        if(cursorEvents.hasNext()){

            while(cursorEvents.hasNext()){

                count++;           

                System.out.println(cursorEvents.next().get("type").toString());
                System.out.println(cursorEvents.next().get("url").toString());
                System.out.println(count);
            }   
        }

        mongoClient.close();
    }   
}

Answer 1

cursor.next() 应该只调用一次，调用它第二次将 return 下一个文档。 documentation

NullPointerException 可能会被抛出，因为下一个文档不存在或 get("url") returns null.

以下代码段应该可以解决这两个问题。

    MongoClient mongoClient = new MongoClient("localhost", 27017);
    MongoDatabase db = mongoClient.getDatabase("behaviourDB_areas");
    MongoCollection cEvent = db.getCollection("event", Document.class);

    MongoCursor<Document> cursorEvents = cEvent
            .find(new BasicDBObject("user_id", "55b20db905f333defea9827f"))
            .sort(new BasicDBObject("timeStamp",1))
            .iterator();

    int count = 0;

    if(cursorEvents.hasNext()) {
        Document doc = cursorEvents.next();
        System.out.println(doc.getString("type"));
        if (doc.containsKey("url")) {
            System.out.println(doc.getString("url"));
        }
        System.out.println(++count);
    }

    cursorEvents.close();
    mongoClient.close();

使用 mongodb 和 java

Using mongodb with java

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